An electrically-driven vehicle, such as a hybrid vehicle or an electric automobile, often employs a system in which DC power from a battery is converted into AC power in an inverter, and the converted AC power is used to drive a motor or a motor generator. Further, the voltage of the battery is often boosted using a boost converter (voltage transformer) and supplied to the motor.
There has been proposed a power supply system which uses such a means for variously changing ON/OFF operation patterns of four switching elements in a boost converter so as to switch connection of a plurality of batteries provided on the low-voltage side between series connection and parallel connection, so that the system is capable of operating in various operation modes, such as a mode for boosting voltage while using the series connection and a mode for boosting voltage while using the parallel connection (for example, refer to JP 2012-70514 A).
In such a power supply system, as the output voltage can be changed over a wide range, by changing the inverter input voltage in accordance with an output from the motor generator, efficient operation of the motor generator can be achieved.
Here, switching elements in the inverter must be prevented from being applied with a voltage higher than the withstand voltage, and also from being operated in such a manner that they overheat. Particularly in a power supply system as described in JP 2012-70514 A, as the output can have a high voltage, it is necessary to reliably protect the switching elements and the like.